This invention relates to a process for operating a pressure swing adsorption (PSA) system for the purification and separation of gases. PSA requires several adsorbers wherein, in one cycle, there are the steps of adsorption, expansion or pressure equalization, purging, and pressure buildup, with the purge gas commonly being the residual gas released during the expansion of adsorbers, and the pressure buildup gas commonly being at least the product gas.
Adsorption processes are frequently utilized because of their economy and efficacy for the purification and separation of gases, for example for the purification of natural gas or for the fractionation of noble gases, air, town gases, cracked gases, hydrogen-rich gaseous mixtures, or gaseous hydrocarbons. In this connection, the PSA methods have proven themselves to be particularly cost effective. In these processes, the desorption and/or regeneration is accomplished by lowering the pressure above the loaded adsorbent. The desorption is completed in the final stages frequently by the use of a purge gas. The gases liberated during the depressurization (expansion) of a loaded adsorber are also utilized more or less completely for raising the pressure in other adsorbers which are at a lower pressure level.
Thus, it has been known, for example, from DOS No. 2,624,346 to use, for the purification or fractionation of several of the aforementioned gaseous mixtures, PSA systems equipped with up to nine adsorbers. The adsorption, effected at a relatively high pressure, is followed, in the known process, by several expansion steps, releasing initially the gas present in the voids between adsorbent particles, and subsequently adsorbed components as well. The first expansion steps are effected by pressure equalization with other adsorbers under lower pressures. Only the gas obtained in the final expansion stage is withdrawn as residual gas, followed by a countercurrent purging or regeneration step to remove, if at all possible, all adsorbed components from the adsorber. Gases obtained during the pressure lowering of adsorbers are used herein as the purge gases. In a number of pressure buildup steps, to a large part by pressure equalization with other adsorbers under higher pressures, the pressure in the desorbed adsorber is then raised again, the last pressure increase to adsorption pressure being conducted with product gas, i.e. the gas withdrawn from the adsorber during the adsorption step.
The adsorbers, conduits, and valves in such plants must be built to withstand severe conditions inasmuch as they are subjected to rapid pressure fluctuations and gas flows. Although this is taken into account in the designing of such plants, damage to and failure of adsorbers are nevertheless sometimes encountered. For example, regarding the adsorber or adsorbers yielding the purge gas, a malfunction results in pressure losses in the adsorbers and in the conduit system, thereby necessitating plant shutdown. Such a shutdown is not only economically disadvantageous per se, but it is also highly disruptive to downstream operations requiring a continuous flow of product or residual gas.